This invention is based on a method of stabilizing a combustion process in which gaseous or liquid residues having little or no calorific value are supplied to a flame in a main combustion chamber in addition to the fuel gas and the air for combustion.
By "gaseous or liquid residues" are meant in this context gaseous or liquid waste materials which are required to be disposed of by combustion. The disposal of residues containing nitrogen entails the problem of nitrogen oxide emissions (NO.sub.x emission), as is well known.
When pressure changes occur in the combustion chamber of a burner as well as rapid changes in load, the flame in many cases breaks off for a short period until it is reignited by means of an accompanying ignition burner. The breaking off and reignition of the flame severely impairs the combustion process and results in an increased CO content and the formation of soot. When liquid waste materials are burned, this may be accompanied by brief smouldering processes which produce a high proportion of undesirable accompanying substances in the flue gas. The basis of all other measures for improving combustion is therefore in the first instance a burner flame which is stabilized under all operating conditions.
In the present state of the art, various methods are used for stabilizing burner flames, depending on the design of the burner and the purpose for which it is used. Baffles in the form of discs or so-called swirlers are frequently employed. Twist stabilized flames are also widely used (mainly for gaseous fuels); the ignition of these flames is maintained by hot flue gas which has been sucked back. A variation used for liquid fuel is based on a built-in solid body over which hot, reabsorbed flue gases flow. This solid body of the burner has the object of preparing the atomized fuel and carrying out a preliminary reaction with the aim of improving the maintenance of the flame and the completeness of combustion. A survey of the structural features and fields of application of industrial gas burners is given in the article by D. Mundus et al in Gas-Warme-International, 37 (1988) 10, pages 509 to 514.
The difficulty of stabilizing a flame increases with decreasing calorific value of the fuel, with variations in load during the working up of waste fuels, e.g. of aqueous emulsions, and with variations in the quality of fuel. The resulting interfering factors and suitable countermeasures are described by B. Lenze et al in VDI-Berichte, 645, pages 269 to 297. It is generally known that a well-stabilized flame provides better combustion than a poorly stabilized flame. Lack of stabilization is generally recognised by an audible and visible pulsation of the flame. External means in the form of a constantly burning pilot flame are frequently used for stabilising liquid fuels which have a low calorific value and fuels with a highly fluctuating calorific value over a wide output range. The pilot flame, which is in many cases produced by a high output pilot burner, is itself extremely stable in its ignition. The ignition of, for example, an atomized low calorie fuel takes place in the part of the spray jet in which the hot jet of exhaust gas encounters the spray jet. As the hot but otherwise substantially inert gas jet of the pilot burner encounters the spray jet from the side, it promotes local evaporation of the droplets, and as the flow continues, the mixture of evaporated fuel with ordinary air as oxygen carrier is ignited if the temperature is sufficiently high, assuming that this mixture is capable of being ignited. The fact that the hot jet of exhaust gas is itself inert is a disadvantage if the fuel gas and air are supplied to the pilot burner in the stoichiometric ratio for obtaining maximum temperatures. Another disadvantage is that the spray jet, which is normally rotationally symmetrical, is ignited only in certain parts. If the substances to be burned are not readily combustible, ignition of the entire spray jet only occurs at a considerable distance further downstream. An oblique flame is then produced which continues to burn in a stable manner after ignition by the pilot flame so long as the liberation of heat in a control volume is sufficient for igniting one or more adjacent control volumes.
When the flame instabilities described above occur and especially when there are locally great turbulences in the flame, there is a risk of increased formation of nitrogen oxides with correspondingly higher emissions of NO.sub.x.